Symbology readers utilize transmitted and reflected light to read symbologies of various kinds. For example, such symbologies may be linear symbologies such as bar codes, or may be multirow or stacked symbologies, such as PDF-417 or Code 49, or multilayer codes. Such symbologies may also include other types, such as "area" symbologies having a matrix of data cells, rather than one or more rows of bars and spaces.
Typically, light is generated by a light source, such as a laser or light emitting diode ("LED") within the reader. The light from the source is directed outwardly through a reader aperture in the reader toward a target object bearing the symbology. Light from the reader aperture strikes the symbology and is reflected. Because the symbology is a pattern of varying reflectances, light is reflected in a pattern corresponding to the reflectance pattern of the symbology. Light reflected from the symbology travels back to the symbology reader where it enters the reader through the aperture. There the reflected light is detected by an optical detector such as a CCD array within the reader. The detector generates an electrical signal corresponding to the pattern of reflectivity of the symbology. This electrical signal is then decoded by a microprocessor and conventional electronics in the reader to determine the information represented by the symbology.
The reader aperture often includes a transparent or color selective plate or window through which the light passes as it travels to and from the symbology. The window protects the interior of the reader from contaminants, such as dirt and moisture. Where the window is color selective, it also acts as a filter to remove light at undesirable wavelengths to improve the operation of the detector.
Ideally, the window transmits light unaffected at the wavelength at which the detector operates. While this may be realized by a clean, transparent or color selective plastic or glass window, such windows often become contaminated or damaged during use. Most commonly, the windows become scratched as the reader is used in the field or contaminants become stuck to an outer surface of the window.
When the window becomes scratched or contaminated, its effectiveness at transmitting light becomes impaired. In some cases, the damage or contamination causes light to be absorbed or reflected. Even where the light is transmitted, it may be diffused or redirected, causing light to miss the detector, or to strike the detector improperly. This problem is especially significant in an area detector type of reader such as a CCD array, where redirected light may strike an improper region of the detector. Such misdirected light can cause the detector to incorrectly indicate the relative reflectances of regions of the symbology, thereby causing the reader to fail to decode the symbology or to decode the symbology incorrectly.
In conventional readers, damage or contamination to the window typically involves replacement of the window. Many times it typically requires the reader to be partially disassembled to remove the window and insert a replacement window. Often this forces the reader to be out of service for an undesirably long period of time. In some cases, it may even involve returning the reader to a service facility.
Another problem encountered with conventional readers is that in some applications the optical properties of the window may be inappropriate for a given application. For example, where a reader is typically used outside in bright light, it may employ a window having a strong filter to reduce the impact of incident light. When the reader is taken to a lower light environment, such as inside of a building, the filter becomes unnecessary and will actually reduce the overall sensitivity of the reader.
In some uses, it may also be desirable to have a symbology reader with a changeable focal plane. For example, where symbologies are readily accessible, such as on small, easily manipulated packages, a short focal length may be advantageous. Where the same reader is used in a less localized application, for example, to inventory large, relatively inaccessible packages in a warehouse, a much longer focal length is advantageous to permit symbologies to be read at a greater distance.
The windows of typical symbology readers are not easily and quickly replaceable, and are not designed with a range of optical properties. Such windows also prevent access to the optics within the reader, making adjustments or adaptations difficult.